It is known (H. K. Muller: Abdichtung bewegter Maschinenteile [Sealing of moving machine parts], Waiblingen, 1990, p. 190, 191) to provide guide rings in hydraulic cylinders for the flush alignment of the rod. On the one hand, they must be sufficiently dimensioned to absorb the guide forces. On the other hand, they must not damage the very finely processed surface of the rod. Instead of the metal guides previously used, guide rings made of materials with a relatively low modulus of elasticity, for example thermoplastic materials, are therefore preferred, which can yield more easily in the event of an inclined position of the rod relative to the cylinder. However, its strength is no longer adequate to withstand the loading which has increased in the course of time. Thermosetting materials are therefore used to a greater extent, but again experience has proved that damage to the rod surface occurs or the guide rings are destroyed.
The invention is based on an analysis of the damage, which shows that the inclined position of the rod relative to the cylinder leads to far higher loading of the guide ring than was previously assumed. Such an inclined position namely results in pressing of the edges which may lead to a peak load far exceeding ten times the nominal load. The force-absorbing capacity of the guide rings also cannot simply be increased by lengthening them in the axial direction or connecting a plurality of guide rings one behind the other because, in an inclined position of the rod, they are load-bearing in any case only on part of their length. In contrast, the problem of the edge load in the case of longer guide rings compared to shorter ones may be further exacerbated because the inclined position of the rod relative to the cylinder causes an additional radial space requirement of the rod at the guide-ring edges, the amount of which is not only proportional to the sine of the inclined position of the rod, but is also proportional to the axial length of the guide ring. Whenever the guide ring which is built into the cylinder and interacts with the surface of the rod is often mentioned here and in the following text for reasons of simplicity, this should also, mutatis mutandis, mean the guide ring which is provided on the piston and interacts with the cylinder tube.
To relieve a piston or a rod guide from the forces originating from an inclined position of the rod, it is known (Patent Abstracts of Japan Vol. 7 No. 183 (M-235) (1328) Aug. 12, 1983--JP-A-58 084 240) to connect a spherical roller bearing between the rod and the piston or between the cylinder and the rod guide. Instead, as is known in another context (FR-A 2 673 449), there could also be spherical mobility of the guide ring. However, the production of spherical surfaces involves substantial expense. This applies even more so because the mutually supporting spherical surfaces have to be of identically spherical design since the pendulum motion taking place between them generally takes place under high forces and therefore a lack of congruence would lead to a high level of wear.